Truss plates are generally employed to join planks of lumber that form floor and roof trusses used in residential housing. Truss plates typically comprise a backing plate and an array of sharp spike-like impaling members, or teeth, that extend outwardly from the backing plate. Adjacent planks of a truss with coplanar surfaces can be permanently joined by pounding or pressing the backing member of a truss plate so that its impaling members penetrate the planks.
Truss plates are typically manufactured by striking out the teeth from a steel sheet with a punch. The punch, which is oriented is essentially perpendicularly to the sheet material, is pressed into the sheet; the punch penetrates and punctures the sheet, thereby forming an aperture therein. The portions of the sheet contacted by the punch remain attached to the remainder of the sheet and are forced by the punch (and usually by a mating female die) into an upright configuration extending away from the sheet. It is these portions extending away from the sheet that form the teeth of the truss plate.
Most commonly, the teeth are formed with a punch having an elongated, generally rectangular cross-section. The aperture formed by such a punch is an elongate rectangle, and the teeth originate from the ends (rather than from the sides) of the aperture.
Truss plate manufacturers are constantly searching for ways to improve the performance of truss plates. In considering truss plate performance, the strength of joints formed by the truss plates, which can be subjected to axial, transverse, shear, and torsional forces, the ease of penetration of the truss plates into planks, and the buckling strength of the tooth (which is important to successful penetration) should be considered. Because the thickness of the truss plate (and its teeth) is often specified by building code or by a particular blueprint, truss plate designers will typically alter the configuration of the tooth when attempting to improve joint strength under different loads and ease of tooth penetration. The final configuration of the teeth depends in large part on the configuration of the punch and the die. An example of a specialized tooth configuration is illustrated in U.S. Pat. No. 3,951,033 to Moehlenpah, which discloses a tress plate having a tooth with an angled, rather than a generally flat, cross-section. One portion of the tooth extends perpendicularly to the longitudinal axis of the punching aperture from which it was formed, and the other portion of the tooth extends at an angle of 135 degrees thereto toward the opposite end of the punching aperture. This tooth is characterized as resisting "rotation relative to the wood members when subjected to axial, nonaxial, and torsional loading." However, in practice truss plates utilizing this tooth proved to be quite difficult to press into wooden planks. Also, because the portion of the sheet steel that forms the angled face of the tooth is forced to move a considerable distance during punching, tooth strength suffers.
A truss plate having similarly angled teeth is disclosed in U.S. Pat. No. 4,418,509 to Moyer et al. The Moyer et al. truss plate differs from that of Moehlenpah in that the angled portions of the teeth disclosed therein narrow to terminate in a sharp side edge. Pairs of teeth are formed with a punch tool that, in cross-section, is a rectangle with two opposite corners being beveled at an angle of 135 degrees relative to the side and end faces of the punch. The female die has a rectangular cross-section with the same opposite corners being beveled at a somewhat sharper angle relative to the side faces of the die. The gap between these angled corners of the punch and die form the narrowed edge of the tooth. However, punching such teeth from sheet steel requires that the portion of the steel that forms the angled section be severely contorted and compressed. As a result, truss plates having these teeth have proven to have low tooth strength.
Others have altered the tooth configuration of truss plates by modifying the cross-sectional profile of the tooth. For example, U.S. Pat. No. 4,639,176 to Smith et at. discloses a truss plate employing pairs of teeth, each of which has a tip portion that extends toward the other tooth of the pair. The theory behind this tooth configuration is that the angled tips improve resistance to the tress plate peeling or being lifted from the wooden planks. Unfortunately, truss plates employing these teeth have been unsuitable due to the tendency of the teeth to buckle when inserted into wooden planks, particularly if the tooth tip contacts an especially rigid portion of the wood grain (such as that of the "winter grain" of the wood). Similar problems are experienced by the truss plates employing teeth with angled tips disclosed in U.S. Pat. No. 3,633,454 to Schmidt et al., and by truss plates employing teeth having a nonlinear backbone. See U.S. Pat. No. 4,318,652 to Gore.
In view of the foregoing, it is an object of the present invention to provide a truss plate having improved resistance to axial, transverse, shear, and torsional loads.
It is also an object of the present invention to provide a truss plate with teeth that easily penetrate wooden planks irrespective of whether the teeth contact soft or rigid portions of the wooden planks.
It is another object of the present invention to provide a punch tool for forming such a truss plate.
It is a further object of the present invention to provide a method for manufacturing such a truss plate.